In a non-reactive saucepan, combine all 4 cups almond milk, 2 tablespoons arrowroot starch, and 1/2 teaspoon agar-agar; whisk or stir until the arrowroot and agar-agar are evenly dispersed and no dry clumps remain.
Heat the mixture over medium heat, stirring constantly, until it reaches a gentle boil (small bubbles form around the edges) and continue stirring for about 30–60 seconds so the agar-agar and arrowroot fully dissolve and the liquid begins to thicken slightly.
Remove the pan from heat and allow the mixture to cool to 100°F–110°F (use an instant-read thermometer or wait until it feels warm but not hot to the touch).
Open 1 vegan probiotic capsule and sprinkle the powder into the cooled milk; add 2 teaspoons pure maple syrup or organic honey and stir gently but thoroughly to combine.
Pour the mixture into clean glass jars, cover, and place the jars in a warm, draft-free location to incubate for 10–12 hours, or until the yogurt has set to your desired firmness.
Transfer the jars to the refrigerator and chill for at least 6 hours before serving.